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77 911S K-Jetronic - Engine Racing Like Pedal Is Floored

chuck barker

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I need help!! I have 6 911's, 67 to 91, so you'd think I could figure this out, but I'm stumped. On way back home from Portland Intl Raceway Rose Cup late this afternoon...3 days of fabulous Porsche racing, but I was not an entrant, just a fan.

In my '77 911S, with K-Jetronic injection (functions flawlessly always) Accelerating on to freeway. suddenly, won't stop accelerating. Managed to get home, by standing on brakes and switching engine off with key.

Examine every moving part from pedal to throttle pedal, lever, cable, side of transaxle case, pivot up to air intake butterfly...all functions perfectly. Removed air cleaner, and the big rubber plemum which takes air from air cleaner chamber over top and feeds down to intake duct. Again, butterfly works perfectly, seats properly. The big valve air regulator below the injector line distributor works fine, lifts up & gently reseats itself. Checked repeatedly this 4 times over. Even tried removing fuel pump fuse to let crank and run dry any residual fuel; it soesnt atrt at all then, but immed starts (and starts racing immediately) as soon as use re-installed.

But...put it back together, try to start, it fires instantly. and you can only allow engine to be on for less than a second...it immediately starts to race as though you have your foot on the floor, but the throttle is still resting in the completely closed position!!!

I had long ago removed thea air pump. So then I tried blocking off, with rubber cover, the air tube that comes up behind the big rubber plenum, thinking it was getting air in that way, but that made no difference.

What is happening? How do I fix this???

Thanks everyone!

Chuck Barker


Tel: 503-847-6360

67 coupe

68 soft rear window Targa

76 911S Targa

77 911S Targa wide body conversion

84 944

91 911 Carrera

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There is a valve for allow extra air to bypass the throttle plate durning cold start/warm up phase. It's called the auxiliary air valve and is located between the intake runner but I can't remember what cylinders. I would remove the valve and plug off both sides of the hoses to see if that corrects your fault.

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There is a valve for allow extra air to bypass the throttle plate durning cold start/warm up phase. It's called the auxiliary air valve and is located between the intake runner but I can't remember what cylinders. I would remove the valve and plug off both sides of the hoses to see if that corrects your fault.

Thanks for the reply. Last night, I had carefully read my Haynes shop manual (which tells you how to take things apart and out them together, but very little about diagnosing what might be wrong), and thought exactly the same as you describe...like maybe air was sneaking into the intake manifold thru this bypass, and "tricking" the flapper modulator valve right below the injector distributor on the big end pf the rubber plenum that extends from the air cleaner chamber over the top to the intake manifold and the pedal-controlled big butterfly valve, and I plugged both ends of that hose (its about 3/4" diameter) with a diaphragm (actually, two layers of a piece of latex hospital gloves), but it made no difference whatsoever! So, any other ideas???


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some additional hints;

The next time the throttle sticks then turn the left food inwards and push it behind te pedal (Or if left foot is on the brake use the right foot). If pushing the pedal upwards "frees" the situation there are some simple points.

1977 should have the Rubber throttle pedal with the internal returnspring, if replaced with the newer Plastic throttle pedal you need a additional spring as return aid device. That spring s mostly placed on the interlinkrod on the trany with a lash under one axle housing bolt. Now the Spring in the rubber peadl base can rust away. The main reason for the spring ist to keep the throttle linkage under tension to avoid ratteling sounds and vibraition to the next uplink on the trany. Ther is a nose barrier in the long rod on the hight from the rear bulkhead that "Barrelroll" isolets the sound but has a slight longtitutinal play. Those barrels age and teh rubber inside comes lose then the play is very high and the lose end ist siting on the outer circumfernece from the barrel making slight sounds and adding some additionl trrootlepaly mostly enough to avoid 100 full throttle on the rear (cost some 10% total power, but aids MPG fuel consumption)

Check if the Pedal stop screw is adjustet correct, if you can overpush the Pedal beound the needed way into the engine bay the throttle rod will bend upwards and the the forward reverse lever will "snap" over beyound his dead center and will not return by itself. This problem is known to 356, 911, 914 so most old mechanics know it yet they hate to crawl down remove the the acessplate and fiddele the interlink out. (Spoils the day)

The thing ist if the Stop is to deep the interlik will be bend all the time, gets weaker and some day breaks off. This problem is often seen if people drive a new SC Plastic Pedal without Stop adjustment, if Stop screw is removed or the complete Pedal Baord is removed or someones swaped the plywoodboard to an aftermarked unit without Stop.

Next thing to check would be; lift the car remove the forward throttle rod from the intermediade link on the trany then pull the upwardlink to throttle intermediatelink on the engine. Sometimes you will find the upwardlink is bend and while pulling the link down the upper end from the rod has a long ball socket wich runs trough the longitutinal cutout from the forward left hand enginebay sheet metal. When you see the rod touching the lower edge from the coutout you will find a postion where it can "hang" on the clampnut for the ball socket or the ball socket itself will bind on the cutout.

Now to fix that you simply have to renew the mostly vanished by aging plastic bushings in the intermedia lever on the engine. If you open the engine bay you will find the alloy bracket with the interlinkrod, just grab it an see if it has play. there should be no more then 1 mm sideways and/or upwards.

The next is on the inner lever you will find a asysmetrical return tension spring with the other end is hanged into a small hook on the undeside from the middle Manifoldrunner (Cyl 2). Those springs sometimes come loose but mostly on an engine job the mechanic didn´t had the patience to fumble that **** coillspring back into to postion.If you have big hands and had a hard day with much coffee to keep yourself "alive" that thing is nearly impossible for right handers. Btw to gain acces to that spot it helps to remove the blowerunit so you can work with both arms on the spot.

There are several ways, one is to run the right arm behind the CIS unit to reach the intermediatelever from backwards grab with a angled longnose plier in the forward end from the tension spring then insert the tension spring into the hook on the manifold and pull the forward end with the plier until you can insert the tension coil into the hole in the lever. A. other way is to insert the tension coil in the lever then run small string from the rear underneath the manifold trough the coil end and pull the coil spring with the string backwards then trey to hit the hook on the manifold and use the finger/s from the other hand to push the coilspring into the correct spot.

However US versions with the mandatory double throttle plate return spring might not need the tension spring as ROW Cars with single throttle return spring so reneing the 2 bushings in the bracket will do it untill the day the engien comes out from the shell (Clutchwork. leaking oil pressure sensor, air leaking manifolds or blown CIS Manifold) then on the workbench installing the tension coil is a puff

Now if anything from the abouve is true i would think about the people who usuallay make the maintenance and repair work to the car.

Those are things who are checked by a Mechanic and on a Testdrive you can feel with your right foot if the Pedalstop ist correct at least high enough to avoid bending the returnlink (You can actually feel how the link is bend). The problem is with mechanics from the newer Generation they never where thought to repair something before it breaks/fails they always wait untill the parts is beyound repair as they either never learnt to repair something as most parts are made to be swaped with a new part and those mechanics know that modern cars are made to survive 5 Years without doing anything but sit and wait untill something fails.

If it fails while warranty then the Car manufactorer/Importer pays the bill, ok they mostly pay flatrate wich deosn´t help macking an income. If the same part fails beyound the factory waranty the flatrate is added by additional costs the Car manufatorer will not pay. So if you run a modern Car Buisness and you have modern mechanics they never learned to lube something to prolong servicelife nor did they repair something at the spot where repair is possible and is on the long run cheaper then renewing the faild unit and have trouble with your costumer stranding somewhere in the jungle.

The other problem is the most young Porsche mechanics learned on watercooled cars and most 911 they worked on where Motronic versions.

Now my problem as a Porsche mechanic is if i see a car wehere even simplest things where not maintained you should spend a day looking trough anything thoe cars are mostly as bag full of potential problems waitung to snap at the rigt time and the right spot. Now on a 911S the first things i would check are the now very expensive alloy front calipers, the condition from the CIS unit and everything relatet to CIS/Ignition as anything wrong on that isse can blow the Manifold, flood the engine with fuel or burn the car down.

However as a fact many costumers aid to the situation if you tell them as a mechanic "that" part ist worn and needs to be attented or fixed they think you will brush them for additional income.Then they drive somewhere else to a other mechanic who tells the thing works it is not new but will last long enough so why touch a perfect runing system ?

1977 CIS should still have the Handthrottlellever wich is just a simple plastic fork puhsing a alloy barrel clamped on the throttlel rod. On very aged cars the plastic fork gets britle and wears down braks off the lever base an tilts around the rod so the lever lays upside down with the end on the middletunnelfloor. Now on full throttle the lever is draged with the rod forward and when releasing the throttle the leverend can stick on the tunnel flour especialy on the sheetmetalstrip that fixes the Brake line there and the barrel stops the throttle somwhere. Tis is a rare occurance i never had seen it peronaly but i know it from an retired race mecahnic ( Those are thouse woundering storys you spend hours finding the culprid ). I guess that only happens once as the chance the fail repeats lowers each time the plastic lever is ramed into the situation. Those hint is just if you can´t find anything and start loosing fate in the withcraft car ;)


(OK if you can´t follow my description then go to Porsche.com and download the 911 74-77 Spare parts Catalog as Pdf then you will find the throttle acctuation splittet into the forward section on maingroup 7 sections 01 forward picture 05 for handlever, section 2 picture 00 and 05 for Pedals and main rod links up into the engine bay wich is shown in maingroup 1 section 07 picture 00 and 05.

BTW Grüsse ain´t my name is Greetings in german

Edited by IXXI
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